1. Field of the Invention
The present invention relates to an image heating apparatus based on an induction heating scheme which heats an unfixed image on a recording medium, and more particularly, to an image heating apparatus effectively applicable to a fixing apparatus for an image formation apparatus such as a copier, facsimile and printer based on an electrophotography scheme or electrostatic recording scheme.
2. Description of the Related Art
As a heating section of an image heating body which heats/fixes an unfixed image on a recording medium such as transfer paper and an OHP (Over Head Projector) sheet, an image heating apparatus based on an induction heating (IH; induction heating) scheme is known.
This image heating apparatus based on an IH scheme generates an eddy current by causing a magnetic field generated by a magnetic field generation section to act on the image heating body and heats/fixes an unfixed image on a recording medium by the image heating body Joule-heated by this eddy current.
The IH-scheme image heating apparatus has the advantages of having a higher heat-generating efficiency than an image heating apparatus using a halogen lamp as a heat source of a heating section which heats the image heating body and being able to shortening a warm-up time. Furthermore, the image heating apparatus using a thin sleeve or belt, etc., as the image heating body has a smaller heat capacity of the image heating body, and can thereby cause the image heating body to generate heat in a short time and improve rising response at the startup considerably.
The magnetic field generation section of this IH-scheme image heating apparatus is constructed of a core made of ferrite or permalloy, an excitation coil wound around the image heating body and a power supply unit which supplies a high frequency current to the excitation coil, etc. The power supply unit is constructed of an inverter circuit, etc., as a feeder circuit that supplies power to a power supply circuit and the excitation coil.
However, in order to avoid misoperation due to overheat, the power supply unit is required to be cooled by a cooling fan, etc., and placed at a position as far as possible from a heat source.
On the other hand, the image heating body is required to be located at a place where it is hardly affected by outside air so that the heating temperature does not become unstable due to cool air, etc., from the cooling fan or the warm-up time is not extended. In order to perform image fixing which is the final step of image formation, the image heating body is inevitably disposed in the vicinity of an ejection port of a recording medium.
Thus, this type of image heating apparatus generally disposes the power supply unit at a place as far as possible from the image heating body so as to supply power from the inverter circuit to the excitation coil through a harness (feeder) (e.g., see Unexamined Japanese Patent Publication No. 2003-347032).
However, the conventional image heating apparatus has a problem that the warm-up time takes a longer time than a time estimated from various preset conditions.
Thus, when the causes for such a problem were investigated, it was discovered that the problem was caused by the harness which electrically connects the excitation coil and the inverter circuit.
That is, as described above, this type of image heating apparatus arranges the power supply unit as far as possible from the image heating body, which causes the length of harness for supplying power to the excitation coil to increase.
Furthermore, since the conventional image heating apparatus extends a litz wire of the coil section of the excitation coil as the harness section as is, the same litz wire as the litz wire of the coil section is used. As the litz wire of the coil section of this excitation coil, a thin litz wire consisting of approximately 40 wires is normally used to increase the number of windings.
However, with such a litz wire with a reduced number of wires, the resistance increases in proportion to the length and a power loss during power supply increases. Such a loss in the litz wire causes the heating efficiency of the image heating body to reduce.
For these reasons, the image heating apparatus using the litz wire as the harness section has a problem that the warm-up time becomes longer than the time estimated from various predetermined conditions.